1. Field of the Invention
The present invention relates to a laser processing method for a gallium arsenide wafer in which a laser beam is radiated to a gallium arsenide (GaAs) wafer along division-intended lines formed on the gallium arsenide (GaAs) wafer, thereby cutting off the gallium arsenide (GaAs) wafer along the division-intended lines.
2. Description of the Related Art
As well known to those skilled in the art, in processes for manufacturing a semiconductor device, a semiconductor wafer is formed in which a plurality of devices such as ICs or LSIs are formed in matrix on a surface of a silicon substrate. The semiconductor wafer thus formed is partitioned along division-intended lines called “streets”, and is cut off along the streets, thereby manufacturing individual semiconductor chips. In addition, a gallium arsenide (GaAs) wafer having a gallium arsenide (GaAs) substrate on a surface of which high-performance devices such as hybrid ICs or high-speed ICs are formed is put to practical use. With regard to a method of cutting off a semiconductor wafer composed of a silicon wafer, or such a gallium arsenide (GaAs) wafer along streets, the following technique is proposed. That is to say, a pulsed laser beam is radiated to a wafer along streets formed on the wafer to subject the wafer to ablation processing, thereby forming laser-processed trenches in the wafer. This technique, for example, is described in Japanese Patent Laid-Open No. Hei 10-305420.
However, when the laser beam is radiated to the wafer along the streets formed on the wafer to carry out the ablation processing, there is newly caused a problem that a heat energy is concentrated on a region to which the laser beam is radiated, thereby generating debris, and the debris are attached on surfaces of devices to reduce a quality of each of chips. In order to solve such a problem due to attachment of the debris, there is proposed a laser processing method of coating a surface to be processed of a wafer with a debris shielding film made of a liquid resin such as polyvinyl alcohol, and radiating a laser beam to the wafer through the debris shielding film. This laser processing method, for example, is described in Japanese Patent Laid-Open No. 2004-188475.
However, when the gallium arsenide (GaAs) wafer is cut off by utilizing the laser processing method described above, there is encountered a problem that a transverse rupture strength of each of the individual chips obtained through the division is reduced. According to experiments made by the inventor of the present invention, when the silicon wafer is cut off by utilizing the above laser processing method, the transverse rupture strength of each of the chips is reduced if the debris are attached on a cut surface of each of the chips obtained through the division. However, it become clear that when the gallium arsenide (GaAs) wafer is cut off by utilizing the above laser processing method, the transverse rupture strength further increases in the case where the debris are attached on the cut surface of each of the chips obtained through the division than in the case where no debris is attached thereon.